1. Field of the Invention
The present invention relates to the structure of the sensing element of a platinum resistance thermometer and the method for manufacturing the same.
2. Description of the Prior Art
Resistance thermometer devices (RTD) are used for measuring temperature by relating the resistance of its "sensing element" to the temperature. An RTD sensing element comprises mainly a circuit made of a metal or alloy whose resistance changes with temperature. The resistance of a sensing element increases in an approximately linear manner with the increasing rate defined as the temperature coefficient of resistance (TCR) of the device. In other words, a RTD with higher TCR is more sensitive than a RTD with lower TCR. It is also well known that the higher the concentration of impurities in the metal or alloy of the circuit, the lower the value of TCR.
Platinum in wire or ribbon form has long been considered as a primary resistance and temperature measuring standard because of its chemical inertness and physical stability. As platinum is used to form the circuit of the sensing element, platinum has another advantage of possessing a high TCR value which increases the sensitivity of the RTD to temperature change. In addition, the resistance of a platinum circuit increases in an almost linear manner with respect to absolute temperature within the range of -200.degree. C..about. 1000.degree. C., whereby the accurate temperature can be easily derived over a wide range of temperature. Therefore, platinum resistance thermometers are well studied and widely utilized. Standards for these platinum resistance thermometers are specifically set forth in JIS C-1604, DIN 43760, and IEC Pub.751, wherein DIN 43760 is generally used as standard which has a standard TCR value 3850 ppm (.degree. C.).sup.-1 of platinum resistance thermometer.
It is to be noted that, when comparing a thin film platinum circuit with a bulk platinum circuit, the TCR value of the former is typically lower than that of the latter, which is called "bulk effect". Therefore, a platinum RTD having a bulk platinum circuit is superior to a platinum RTD having a thin film platinum circuit in its sensitivity.
However, the conventional platinum RTDs are relatively expensive, not only because of the expansiveness of platinum, but also because of the high manufacturing costs of platinum sensing elements. A conventional platinum RTD is typically made by forming the circuit pattern of platinum sensing element on the surface of a dielectric layer. Unfortunately, pure platinum exhibits poor adhesiveness to most practical dielectric materials, and the platinum circuit pattern deposited on the surface of dielectric substrate may easily be detached from the surface. Some dielectric materials exhibit good adhesiveness to platinum, but still suffer from their own drawbacks. For example, the silicone substrate is preferred in that it is comparatively cheap, with a good smoothness, and can be easily processed, however, an platinum-silicon alloy is formed during the heat-treatment at high temperature, thus resulting in a problem related to the characteristics of sensing element. The silicon dioxide substrate is relatively cheap, but has the drawback that no sufficient adhesion with respect to platinum can be provided. The alumina substrate is inexpensive and heat-resistant and with superior adhesion to platinum, but its rough surface leads to difficulties in the formation of fine pattern. Although the surface may be smoothed by surface polishing, the polishing of the alumina substrate having a large hardness results in an extreme cost increase in the substrate material. The sapphire substrate is superior in heat-resistance, adhesion with respect to platinum, and surface smoothness; however, it is very expensive and it is difficult to be cut into small chips. Accordingly, typical platinum resistance thermometers are manufactured by forming a platinum pattern on the surface of a dielectric material using specially developed fabricating processes and/or equipment, thereby greatly increases the manufacturing cost.
To solve the aforementioned problems, a platinum RTD is disclosed in U.S. Pat. No. 4,129,848. As shown in FIG. 1A, a layer of silicon dioxide 12 is grown on the upper surface of a clean silicon substrate 11 by heating substrate 11 in an oxygen-containing atmosphere. The exposed surface of silicon dioxide layer 12 is roughened by sputtering etch to produce many microscopic pits or holes extending from the exposed surface downward but not so far as substrate 11 (FIG. 1B). Subsequently platinum layer 13 is deposited onto the surface of silicon dioxide layer 12 by sputtering deposition using a two-step process. This roughened interface between the silicon dioxide layer 12 and the platinum layer 13 increases the adhesion of the platinum layer 13 to silicon dioxide layer 12 (FIG. 1C). A quartz layer is sputtering deposited over the platinum layer 13, and then coated with a photoresist mask for chemical etching. The chemically etched quartz layer forms a quartz mask 14 having a positive pattern identical to the desired platinum circuit pattern (FIG. 1D). The exposed platinum layer 13 and part of the quartz mask 14 are then sputtering etched away, leaving the platinum protected by the quartz mask 14 in a predetermined pattern, that is, platinum circuit pattern 15. Subsequently the quartz mask 14 is removed and further procedures like heat treatment are proceeded. However, sputtering etch process tends to introduce impurities and cause the loss of definition at the edges of the platinum pattern. In detail, sputtering etch of the silicon dioxide causes deposition of the silicon dioxide molecule into the platinum as impurity, and the platinum structure is affected at its edges so that there is loss of definition. Besides, the exposed surface of the silicon dioxide layer 12 may also be etched away because of the poor selectivity of sputtering etch procedure. As mentioned earlier, a highly pure platinum circuit is required for maintaining the TCR value of a platinum sensing element, the introduced impurities changes the TCR value of the thermometer, thus dramatically affects the accuracy of the platinum RTD.
In a commercialized platinum thermometer, platinum circuit is typically disposed on the surface of a dielectric substrate like alumina having excellent adhesion to platinum. The U.S. Pat. No. 4,805,296 discloses a method for manufacturing platinum resistance thermometer, in which a platinum layer is sputtering deposited on an alumina film located on the surface of a silicon substrate, and the platinum circuit pattern is formed by sputtering etch as well. However, use of mask to etch away the platinum film in unwanted areas causes a problem that the pattern cannot reasonably be defined with desired precision and uniformity for obtaining a small size and close spacing of the strip section. In addition, the mask layers tend to deteriorate before the etching process is complete and impurities tend to be introduced into the platinum. Therefore, most patents relating to the platinum resistance thermometer (e.g., U.S. Pat. No. 4,050,052, 4,103,275, 4,469,717, 4,627,902, and 4,649,364) describe only the processing conditions for treating the platinum and substrate, while the detailed process for forming the platinum pattern are absent.
A method for forming a platinum resistance thermometer without using a sputtering-etch mask is disclosed in U.S. Pat. No. 5,089,293, in which an alumina (or sapphire) substrate having excellent adhesiveness to platinum is employed. Referring to FIG. 2A, a silicon dioxide layer 22, which is called a liftoff medium, is deposited on the upper surface of substrate 21. A photoresist pattern 24 is formed after a desired path pattern has been exposed on the photoresist and the photoresist is developed, leaving the strip pattern in the photoresist over the surface of silicon dioxide layer 22 (FIG. 2B). The underlying silicon dioxide layer 22 is chemically etched in the areas not protected by photoresist pattern 24, thereby defines a desired path, that is, a positive pattern, on the surface of the substrate 21 for deposition of a platinum circuit pattern (FIG. 2C). After the photoresist pattern 24 has been completely removed from the remaining negative patterned silicon dioxide layer 22, the substrate 21 is ready for deposition of platinum (FIG. 2D). Platinum is then sputtering deposited on the negative patterned silicon dioxide layer 22 and on the exposed surface of substrate 21 to form a platinum layer 23, as shown in FIG. 2E. The silicon dioxide layer 22 has a thickness of at least 1.3 to 1.5 times that of the platinum layer 23. Referring now to FIG. 2F, an interconnecting section 23A, located on the side surface of silicon dioxide layer 22, interconnects between the platinum layer on the silicon dioxide layer 22 and the platinum layer on the substrate 21. The deposition condition is carefully controlled so that the interconnecting section 23A forms a porous thin-film structure. The interconnecting section 23A is sufficiently porous so that an etching solution will pass through the thin film. Hydrofluoric acid is then added to etch away the remaining parts of the silicon dioxide layer 22 within the intersectional space between the substrate 21 and the platinum layer 23. After etching all of the silicon dioxide layer 22 away, the portions of the platinum layer located on top of the silicon dioxide layer may be mechanically separated from the portions of the platinum layer deposited on the surface of substrate 21 in the region of the interconnecting section 23A. The platinum layer deposited on the surface of substrate 21 is tightly bond to the substrate surface, thus a platinum circuit pattern 25 of a resistance thermometer is formed on the surface of substrate 21 (FIG. 2G). All the materials and processing procedures are carefully chosen such that contamination to platinum is avoided.
According to the aforementioned method disclosed in U.S. Pat. No. 5,089,293, a platinum resistance thermometer having defined platinum pattern and pure platinum circuit can be obtained. However, because of their hardness, the alumina or sapphire substrate used in this prior art is difficult to handle, while the subsequent processing and further treatments are also hard to proceed. Besides, etching a hindered liftoff medium away through the porous thin platinum film requires some specialized manufacturing processes and equipment, thereby greatly increases the fabricating cost.
From the aforementioned prior arts, one can finds that the price of a platinum RTD sensing element can be lowered when the fabricating cost of this platinum RTD sensing element is greatly reduced by batch producing the platinum sensing element having purest platinum circuit with the typical procedures and equipment commonly used in the semiconductor industry. In addition, the platinum RTD sensing element and other integrated circuits can be formed on a single chip if a silicon substrate is used, thereby reduces the size and simplifies the assembling procedures of RTD.